1. Field of the Invention
This invention relates to a pressure-difference type mass flowmeter with constant volumetric recirculation of the fluid to be measured.
2. Description of the Prior Art
As is well known, there have been contrived and actually used a variety of mass flowmeters of the type mentioned just above. Some of them require two positive displacement pumps of the exactly identical capacity, while others make do with a single pump but instead require two or four orifices to be installed in the flow path of the meter. Some require two pressure difference transducers for reading out the flow rate, while others do with single such transducer. With the recirculation always kept in constant volumetric rate, some flowmeters require, when the flow rate to be measured skips from a predetermined range to another, switching-over of the points to which the pressure difference transducer is connected; while others can anyhow give the readout with fixedly connected transducer or transducers, throughout the entire variation range of the flow rate to be measured, from zero to maximum.
Such conventional flowmeters have therefore drawbacks in that the effort for reducing the number of any one of the three groups of the component parts: pumps, orifices and difference-pressure transducers; is inevitably to lead to increasing the number of other group or groups, thus resulting at any rate in complicated overall structure. Drawbacks inherent in orifices are big pressure loss, causing waste power consumption. Moreover, orifices are apt to gather dirt, and the clogging, even though partially, will cause considerable fluctuations in their flow characteristics. Still further drawbacks are generally seen of the conventional flowmeters in that the linearity between the pressure difference readout and the mass flow rate to be measured is affected by the fluctuation of the resistance coefficient of the flow path in general, in consequence of variation in Reynolds number, namely variation in the flow rate to be measured itself (though alleviation of such fluctuation, thus rectification of non-linearity, is the main object of installing the orifices in spite of their inherent drawbacks as commented above; flow coefficient of the orifices considerably fluctuates under very wide variation in Reynolds number) and also the sensitivity of the flowmeter will fluctuate as the temperature varies.